It is well known in the pipeline industry that pipelines are subject to defects such as corrosion, mill defects, third party damage (e.g. dents, scratches, gouges), stress corrosion cracking and hydrogen induced cracking. Defects have the potential to cause catastrophic failure in pipelines that are in operation or under testing.
Various internal and external inspection methods for pipelines are well known in the art. When a defect has been identified, one of several prior art methods of repair may be selected based on the location of the pipeline, the type of defect and size of defect. David Boreman, Bradley Wimmer and Keith Leewis have published a paper on selection of repair methods titled “Repair Technologies for Gas Transmission Pipelines” in the Pipeline & Gas Journal in March 2000. The subject article is incorporated herein by reference. Additionally, a discussion of known prior art repair equipment and systems is compiled in a paper prepared by AEA Technology Consulting for the Health and Safety Executive Division for Offshore Technology Report 2001/038, the disclosure of which is incorporated by reference herein.
Stress Crack Corrosion (“SCC”)
Particularly problematic and difficult to repair are defects in pipelines caused by stress crack corrosion (“SCC”). SCC is an area of corrosion (in this case on a pipe) that has developed cracks; the cracks increase in severity as the pipeline flexes due to operating pressure changes. The continuous flexing causes the metallic structure of the pipe to become brittle until there is a catastrophic fatigue failure. It is known within the industry that sleeves can be used for the repair of pipelines. However, there is a definite need for a practical, dependable, cost-effective, reliable, and repeatable product/method for the specific and permanent repair of pipelines having Stress Crack Corrosion (SCC). To date, the only proven and positive method to renew or rehabilitate pipes with SCC is to shut down the pipeline, depressurize the pipe, remove, vent or dump the product that the pipeline is carrying, cut out the section of pipe with SCC and replace that section of pipe with good or new pipe. It is then necessary to reinstate or commission the pipe to service using cumbersome and expensive pressure testing and commissioning procedures.
Prior Art
U.S. Pat. No. 5,722,463 discloses a pipe reinforcing sleeve that purports to be suitable for repairs of SCC defects in pipelines. The patent discloses heating the sleeve with a torch to thermally expand the sleeve. While heated and in an expanded state two halves of the sleeve are joined by welding. Upon subsequent cooling, the sleeve will be in tension and apply a compressive force to the pipeline. The system of the '463 patent requires a very tightly controlled installation method, including heating of the repair sleeve to a pre-determined temperature prior to installation, and maintaining this temperature during installation. Under field conditions this may present considerable practical difficulty. Under field conditions, such as fluctuating environmental temperatures, fluctuating pipeline flow rates, and because the pipeline is a large and efficient heat sink, repeatable repair results may be difficult to achieve.
U.S. Pat. No. 5,345,972 discloses a method of repairing a pipeline by applying a cylindrical sheath cladding (sleeve) consisting of two half shells welded together to create an inter-space, which is sealed by applying gaskets. Catalyzed resin is injected into the annular space. The sheath wall is placed under tension when the filling material is injected and the pipeline wall is put into compression. The gaskets sealing the ends of the pipe are removed after the epoxy has set. Due to shrinkage of the resin upon setting, the tension in the shell wall and the compression in the pipeline by the catalyzed resin is reduced or eliminated. After the gaskets sealing the ends of the pipe are removed, pressure is not maintained in the annulus between the sheath (sleeve). Because the annulus is not pressure sealed, the portion of the pipeline containing the defect can flex with changes in internal operating pressure. The method of repair taught in the '972 patent is not suitable for repair of SCC damage and there is no teaching of use for SCC repairs in the patent.
No single prior art pipeline system is versatile enough to repair all types of defects. A versatile system of pipeline repair is needed that includes the ability to permanently repair and rehabilitate in service pipelines, especially those with an SCC defect.
Distinguishing Features of the Present Invention
Some of the distinctive features of the pipeline repair system of the present invention are its cost effectiveness, reliable performance, straightforward installation procedure, and the fact that the end result is provable and scientifically documented as a solution to the problem. Particularly distinguishing the present invention from the prior art, the system can be used to universally repair all types of defects in a pipeline including repair of Stress Corrosion Cracks (SCC). As discussed in the “Repair Technologies for Gas Transmission Pipelines” article, most other repair methods including composite repair sleeves are restricted to repair of cracks of length less than 40% of the pipe wall thickness. The sleeve of the present invention becomes an integral permanent part of the pipeline and is capable of carrying a majority of the stress load of the pipeline.